聚合體囊胞的結構與機械特性

Structural and Mechanical Characteristics of Polymersomes

張弘郁(Hung-Yu Chang)

指導教授 ： 諶玉真

國立臺灣大學/工學院/化學工程學研究所/博士(2014年)

https://doi.org/10.6342/NTU.2014.02962

摘要

聚合體囊胞是由雙親性高分子所構成，它的機械、輸送性質，以及融合行為可藉由改變囊胞的形狀、大小、和膜厚等結構特徵來調控。如此多變的性質和結構的穩定性，讓聚合體囊胞受到廣泛地矚目。儘管許多實驗技術已投入聚合體囊胞的研究，微觀的結構樣貌與性質的量測仍飽受限制。介觀尺度的模擬方法可以補足實驗在微觀尺度上所達不到的限制，因此能提供一套可行的方法，幫助了解聚合體囊胞結構型態與其物化性質間的關聯性。此外，模擬方法的預測性也將有助於聚合體囊胞未來的發展與應用性的開發。
本論文利用耗散粒子動力學法(DPD)來對三種高分子系統進行研究。除了建立了不同分子結構的高分子一系列的結構性相圖之外，我們特別針對聚合體囊胞進行微觀性質與其生物性行為的研究。此論文分為三個部分，第一部分(第三章)探討具有疏溶劑性主鏈段，搭配兩種側鏈，一條為親溶劑性的同聚型分子與另一條雙親性的雙嵌段分子，所構成的刷狀型高分子的自組裝行為研究。依據各鏈段長度、分子結構、與主鏈上的接枝密度可產生多成份的聚集型態，其中包含五種囊胞、多孔型聚集體、蟲狀微胞、甜甜圈狀微胞、漢堡狀微胞與寡分子聚集性微胞。在特定條件下的刷狀高分子，可以形成非對稱性多層形態囊胞的聚集結構。當此刷狀高分子具有溫度反應性的鏈段時，溫度的變化將能誘發囊胞結構的轉變，讓聚合體囊胞由非對稱性四層結構轉為對稱性七層結構。因此，當溫度超過刷狀高分子的最低臨界溶解溫度時，囊胞也會伴隨溫度的提升而縮減尺寸。此模擬結果也與實驗上觀測到的現象一致。當刷狀高分子內的成分比例固定時，其聚集形態仍可藉由改變各鏈段排列方式而改變，尤其當兩種親溶劑鏈段分別接枝在主鏈與側鏈段上時，非對稱性囊胞的聚集形態才容易出現。模擬結果也發現，當刷狀高分子主鏈段上的接枝密度太高會促使形成甜甜圈形態的微胞，密度太低則容易出現漢堡狀微胞，而非對稱性囊胞則只能在適當的接枝密度下形成。
論文的第二部分(第四章)，研究具有酸鹼反應性的親水性支鏈段接枝在疏水性主鏈上的雙親性梳狀高分子，在選擇性溶液下的自組裝行為。藉由改變溶液酸鹼性、高分子濃度、支鏈長度、與接枝密度可產生豐富的聚集形態。根據結構性相圖發現，微胞聚集容易在酸性環境下生成，而隨著鹼性略微地增加，單層膜囊胞結構(ULV)將能被觀測到，而其尺寸還會隨著高分子的濃度增加而增加。當溶液的酸鹼度接近中性時，更有機會看到有如洋蔥般一層層結構的多層膜囊胞(MLV)的形成。囊胞膜的層數更能隨著高分子的濃度上升而增加。此模擬結果也與實驗上所觀察到的現象一致。模擬結果還發現，當接枝密度降低，MLVs的疏水層厚度明顯增加，而總囊胞膜層數則是變少。主鏈段上的接枝密度降低或溶液趨近中性卻也會降低水分子在ULV之間的通透性。階段性藥物釋放研究，是將兩種親疏水性的藥物放置於MLV中的不同膜層並觀測藥物的釋放能力。結果顯示，不論藥物擺放位置，親水性的藥物平均而言都比疏水性的藥物來的較快被釋放。梳狀高分子形成的囊胞擁有與微脂粒(liposomes)類似的融合行為，然而卻遵循不一樣的融合機制。梳狀高分子形成的ULVs，其融合機制遵守非等向性莖稈-孔洞蔓延程序(anisotropic stalk-pore scenario)，在進行融合的囊胞接觸區邊緣處，梳型高分子具有強大的形變，因此莖稈與孔洞也都偏愛在該處生成。MLVs的融合行為則是一層層進行，而內層融合速度較外層來的快。當控制溶液接近中性，可以加速囊胞的融合行為，而高分子支鏈長度的增加卻反而阻礙融合的進行。最後值得一提的是， MLVs的融合有可能會形成比未融合前還要更多膜層的MLV。
第三部分的論文(第五章)則是探討具有非極性/極性/非極性(n-p-n')樣式的高分子改質之奈米球在選擇性溶液下的自組裝行為。在極端疏水性的奈米球(n-part)上接枝上兩條疏水性的高分子(n'-part)，而奈米球表面只有一個小區塊(p-part)具有親水的特性。藉由改變接枝高分子的疏水性與長度，可控制產生出三種型態的聚集結構，包含奈米球囊胞，核-殼型微胞與嵌段式蟲型聚集體。奈米球囊胞可以在高分子一定範圍的長度下形成，然而高分子的存在卻會影響奈米球在囊胞膜中雙層排列的規整性。囊胞膜結構特性，包含規整性，厚度，以及內層膜單位面積上的奈米球密度，都隨著接枝上的高分子長度增加而下降。這些結果都顯示，長鏈段高分子的存在會促使奈米球在囊胞膜內的交錯排列，如此將有助於減緩內層膜中過度壅擠的高分子。此外，奈米球囊胞的輸送與機械性質也已被探討。由於奈米球在囊胞膜內的排列會因高分子的存在而鬆散，因此隨著高分子長度的增加，水分子的通透性也跟著增加。相反地，囊胞膜的張力卻因高分子立體結構所形成的屏障，讓極度疏水的奈米球與溶劑接觸降低而下降。儘管，囊胞的融合行為有助於囊胞張力的舒緩，在我們的系統之中並沒有觀測到奈米球囊胞的融合現象。除此之外，許多在微脂粒發現與尺度相關的性質並沒有在奈米球囊胞中被觀測到，推測此原因與奈米球等向性球型結構有關。

關鍵字

雙親性高分子 ； 自組裝行為 ； 耗散粒子動力學法(DPD) ； 聚合體囊胞 ； 環境反應性

並列摘要

Polymersomes self-assembled from amphiphilic macromolecules have attracted a growing attention because of their multifunctionality and stability. By controlling the structural characteristics of polymersomes, including vesicle shape, size, and membrane thickness, the mechanical and transport properties as well as the fusion behavior of a polymersome can be manipulated. Numerous experimental techniques have been developed to explore polymersome characteristics; however, experimental microscopic observations and knowledge of vesicles are limited. Mesoscale simulations can complement experimental studies of the vesicular features at the microscopic level and thus provide a feasible method to better understand the relationship between the fundamental structures and physicochemical properties of a polymersome. Moreover, the predictive ability of the simulation approaches may greatly assist developments and future applications of polymersomes.
This dissertation uses dissipative particle dynamics (DPD) to explore the self-assembly of three polymeric systems. A series of morphological phase diagrams of polymers with different topological structures have been constructed. In addition, we have paid particular attention to the fundamental properties of polymersomes and their biological behaviors. There are three parts in this dissertation. The first part (Chapter 3) investigates the self-assembly of polymer brushes consisting of a solvophobic backbone attached with two different side chains, solvophilic and amphiphilic diblock. Dependent on the block length, molecular architecture, and grafting density, the multicompartment aggregate exhibits a rich variety of morphological conformations, including five types of vesicles, porous aggregates, worm-like micelles, donut micelles, hamburger micelles, and unimolecular micelles. For certain polymer brushes, atypical polymersomes with asymmetric multilayered membranes are spontaneously formed. Moreover, temperature variation induced morphological transformation from an asymmetric four-layered polymersome to a symmetric seven-layered polymersome is observed for polymer brushes containing a thermoresponsive block. Consequently, the resulting polymersome decreases in size quite sharply as temperature exceeds lower critical solution temperature. These simulation findings are consistent with experimental observations. At a fixed composition of polymer brushes, the aggregate morphology varies with the structural arrangement of the two solvophilic blocks in the molecule. Asymmetric polymersomes are formed when the two solvophilic blocks are separately attached to the backbone and side chain. Although asymmetric vesicles are observed at moderate grafting density, unique donut aggregates are formed for high density but hamburger micelles develop at low density.
In the second part (Chapter 4), the self-assembly behavior of amphiphilic comb-like graft copolymers bearing pH-responsive hydrophilic side chains on hydrophobic backbone in a selective solvent is studied. The aggregate exhibits a rich variety of the morphological conformations dependent on pH, polymeric concentration, side chain length, and grafting density. The morphological phase diagram shows that micellar aggregates take shape at low pH. As pH increases, unilamellar vesicle (ULV) can form at low polymer concentration and the vesicle size grows with increasing concentration. Further increment of pH to neutral leads to the formation of multilamellar vesicles (MLV) with the layer-by-layer structure similar to that of an onion. The total number of layers rises with increasing polymer concentration. Our simulation outcomes are consistent with the experimental observations. The simulation results also reveal that as the grafting density is decreased, the thickness of the hydrophobic layer grows and thus the total number of layers declines for MLVs. The water permeation process through ULV descends as the grafting density is decreased or the neutral pH is approached. Controlled releases of two types of drugs with different hydrophobicity situated at different layers of MLV are also examined. The release rate of hydrophilic drug is faster than that of hydrophobic drug. Polymersomes tend to fuse but the fusion mechanism is different from that of liposomes. The fusion pathway of ULVs follows the anisotropic stalk-pore scenario and both stalk and pore formation prefer to occur on the edge of the contact zone due to significant deformations of comb-like polymers in this region. The fusion pathway of MLVs is layer-by-layer and the fusion time of the inner layer is faster than that of the outer layer. The fusion process is expedited at about neutral pH but is prolonged substantially as the side chain length is increased. Finally, it is interesting to observe that the total number of layers of the fused MLV can be greater than that of MLVs before fusion.
In the third part (Chapter 5), the self-assembly behavior of polymer-tethered nanoballs (NBs) with nonpolar/polar/ nonpolar (n-p-n')motif in a selective solvent is investigated. A model NB bears two hydrophobic polymeric arms (n'-part) tethered on an extremely hydrophobic NB (n-part) with hydrophilic patch (p-part) patterned on its surface. Dependent on the hydrophobicity and length of tethered arms, three types of aggregates are exhibited, including NB vesicle, core-shell micelle, and segmented-worm. NB vesicles are developed for a wide range of hydrophobic arm lengths. The presence of tethered arms perturbs the bilayer structure formed by NBs. The structural properties including the order parameter, membrane thickness, and area density of the inner leaflet decrease with increasing the arm length. These results indicate that for NBs with longer arms, the extent of interdigitation in the membrane rises so that the overcrowded arms in the inner corona are relaxed. The transport and mechanical properties are evaluated as well. As the arm length grows, the permeability increases significantly because the steric bulk of tethered arms loosens the packing of NBs. By contrast, the membrane tension decreases owing to the reduction of NB/solvent contacts by the polymer corona. Although fusion can reduce membrane tension, NB vesicles show strong resistance to fusion. Moreover, the size-dependent behavior observed in small liposomes is not significant for NB vesicles due to isotropic geometry of NB.

並列關鍵字

amphiphilic polymer ； self-assembly ； dissipative particle dynamics (DPD) ； polymersome ； stimuli responsiveness

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